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A Galactic Plane Defined by the Milky Way HII Region Distribution

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A Galactic Plane Defined by the Milky Way HII Region Distribution Draft version December 7, 2018 Typeset using LATEX twocolumn style in AASTeX62 A Galactic Plane Defined by the Milky Way H II Region Distribution L. D. Anderson,1, 2, 3 Trey V. Wenger,4 W. P. Armentrout,1, 3, 5 Dana S. Balser,6 and T. M. Bania7 1Department of Physics and Astronomy, West Virginia University, Morgantown WV 26506, USA 2Adjunct Astronomer at the Green Bank Observatory, P.O. Box 2, Green Bank WV 24944, USA 3Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA 4Astronomy Department, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325, USA 5Green Bank Observatory, P.O. Box 2, Green Bank WV 24944, USA 6National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville VA, 22903-2475, USA 7Institute for Astrophysical Research, Department of Astronomy, Boston University, 725 Commonwealth Ave., Boston MA 02215, USA ABSTRACT We develop a framework for a new definition of the Galactic midplane, allowing for tilt (θtilt; rotation ◦ ◦ about Galactic azimuth 90 ), and roll (θroll; rotation about Galactic azimuth 0 ) of the midplane with respect to the current definition. Derivation of the tilt and roll angles also determines the solar height above the midplane. Here we use nebulae from the WISE Catalog of Galactic H II Regions to define the Galactic high-mass star formation (HMSF) midplane. We analyze various subsamples of the WISE catalog and find that all have Galactic latitude scale heights near 0:30 ◦ and z-distribution scale heights near 30 pc. The vertical distribution for small (presumably young) H II regions is narrower than that of larger (presumably old) H II regions (∼ 25 pc versus ∼ 40 pc), implying that the larger regions have migrated further from their birth sites. For all H II region subsamples and for a variety of fitting methodologies, we find that the HMSF midplane is not significantly tilted or rolled with respect to the currently-defined midplane, and therefore the Sun is near to the HMSF midplane. These results are consistent with other studies of HMSF, but are inconsistent with many stellar studies, perhaps due to asymmetries in the stellar distribution near the Sun. Our results are sensitive to latitude restrictions, and also to the completeness of the sample, indicating that similar analyses cannot be done accurately with less complete samples. The midplane framework we develop can be used for any future sample of Galactic objects to redefine the midplane. Keywords: Galaxy: structure { ISM: H II regions 1. INTRODUCTION the Galaxy have found an asymmetry in the distribu- The midplane, the plane at Galactic latitude b = tion of sources above and below the plane, with more 0 ◦, was defined in 1958 by the IAU subcommission sources found below the IAU plane than above it. This 33b, which set the Galactic coordinate system (Blaauw asymmetry is generally assumed to be the result of the et al. 1960). The IAU midplane definition comes from Sun's location above the IAU Galactic midplane. the Galactic Center location in B1950 coordinates of Previous studies of the vertical distribution of objects and solar height above the plane can be categorized as arXiv:1812.02244v1 [astro-ph.GA] 5 Dec 2018 (17:42:26.6, −28:55:00) and the north Galactic pole lo- cation in B1950 coordinates of (12:49:00, +27:24:00). either using stellar or gas samples. Solar height stud- Ideally, the midplane definition would contain the mini- ies are summarized in Humphreys & Larsen(1995) and mum of the Galactic potential and there would be equal Karim & Mamajek(2017). Studies of stellar samples amounts of material above and below the midplane. The have a long history; perhaps the first such study was vertical distribution of objects with respect to the Galac- done by van Tulder(1942), who found an asymmetry tic midplane tells us fundamental parameters of Galactic in the stellar distribution that implied that the Sun is structure, such as the scale height of the objects studied, 14 ± 2 pc above the plane. Typical stellar studies ex- amine discrepancies in the number of sources toward the Sun's height above or below the midplane, z , and even the orientation of the midplane itself. Nearly all the north and south Galactic poles to determine the so- previous studies of the vertical distribution of objects in lar height (e.g., Humphreys & Larsen 1995). A typical 2 Anderson et al. Table 1. Previous Results Analyzing the b and z-Distributions of Galactic Objects Tracer Galactic Latitude b Height z Scale Heighta Peak Scale Heighta Peak Zone Referenceb (deg.) (deg.) (pc) (pc) ATLASGAL 870 µm cont. 0:3 −0:076 ± 0:008 −60 ◦ < ` < 60 ◦; j b j ≤ 1:5 ◦ 1 ATLASGAL 870 µm cont. 28 ± 2 −6:7 ± 1:1 −60 ◦ < ` < 60 ◦; j b j ≤ 1:5 ◦ 2 ATLASGAL 870 µm cont. 31 ± 3 −4:1 ± 1:7 −60 ◦ < ` < 0 ◦; j b j ≤ 1:5 ◦ 2 ATLASGAL 870 µm cont. 24 ± 1 −10:3 ± 0:5 0 ◦ < ` < 60 ◦; j b j ≤ 1:5 ◦ 2 BGPS 1.1 mm cont. −0:095 ± 0:001 −10 ◦ < ` < 90:5 ◦; j b j < 0:5 ◦ 3 BGPS 1.1 mm cont. 27 ± 1 −9:7 ± 0:6 15 ◦ < ` < 75 ◦; j b j < 0:5 ◦ 4c IR-identified star clusters 0:66 ± 0:07 Entire Galaxy 5 Ultra-compact HII regions Entire Galaxy 6 Ultra-compact HII regions 31 −10 ◦ < ` < 40 ◦; j b j ≤ 0:5 ◦ 7 Ultra-compact HII regions 0.6 30 Entire Galaxy 8 HII regions 42 −11 17:9 < ` < 55:4 ◦; j b j < 1 ◦ 9 d HII regions 39.3 −7.3 Entire Galaxy, RGal < R0 10 e HII regions 33:0 ± 0:06 −7.6 Entire Galaxy, RGal < R0 11 High mass star forming regions 29 ± 0:5 −20 to 0 17:9 < ` < 55:4 ◦; j b j < 1 ◦ 12 ◦ ◦ CH3OH masers 0:4 ± 0:1 −174 < ` < 33 13 HI cold neutral medium ∼ 150 Entire Galaxy 14 CO 0:45 Entire Galaxy 15 Far-IR dust 0:50 Entire Galaxy 16 Far-IR dust 0:32 −0:06 −70 ◦ < ` < 68 ◦; j b j < 1:0 ◦ 17 158 µm [CII] 0:56 73 −28 Entire Galaxy at b = 0 ◦ 18 aAs listed in the paper, regardless of whether value corresponds to the exponential or Gaussian scale height (see Equation1). b1: Beuther et al.(2012); 2: Wienen et al.(2015); 3: Rosolowsky et al.(2010); 4: Ellsworth-Bowers et al.(2015); 5: Mercer et al.(2005); 6: Bronfman et al.(2000); 7: Becker et al.(1994); 8: Wood & Churchwell(1989); 9: Anderson & Bania(2009); 10: Paladini et al.(2004); 11: Bobylev & Bajkova (2016); 12: Urquhart et al.(2011); 13: Walsh et al.(1997); 14: Kalberla(2003); 15: Dame et al.(1987); 16: Beichman et al.(1988); 17: Molinari et al.(2015); 18: Langer et al.(2014) cThese authors correct for the solar offset above the Galactic midplane. Quoted values are our computations from their database, using z = d sin(b) and the Brand(1986) rotation curve. dValues are only for the sample with the most accurate distances. eListed scale height is from the authors' Gaussian model fit. value for the solar height from stellar studies is 20 pc; The tracers that are most sensitive to HMSF have nar- for example, Ma´ız-Apell´aniz(2001) used OB stars from row distributions with scale heights ∼ 40 pc, whereas Hipparchos to derive z = 24:2 ± 1:7 pc, Chen et al. the distributions of H I, CO, far-infrared emitting dust, (2001) used stars from an early release of the Sloan Dig- and C II are broader. ital Sky Survey (SDSS) to derive z = 27 ± 4 pc., and The solar height above the plane derived using gas Juri´cet al.(2008) found using SDSS data release 3 (with tracers is generally lower than that found from stellar some data release 4) that the z-distributions for stars of tracers (see compilation in Karim & Mamajek 2017). a range of colors and brightnesses are all consistent with Typical values are near 10 pc. For example, Bobylev & z ' 25 pc. Bajkova(2016) found z = 8 ± 2 pc from a sample of We summarize the studies of Galactic latitude and H II regions, masers, and molecular clouds and Paladini z-distributions that use gas tracers in Table1, focus- et al.(2003) found z = 9:3 ± 2 pc using a sample of ing on works that use tracers sensitive to HMSF. This H II regions. table contains the peak and scale height of the distri- Because it was defined using low-resolution data and butions. If the fits were exponential, we list the stated our measurements have since improved significantly, the scale height. If the fits were Gaussians, we list the scale IAU-defined Galactic midplane may need to be revised height h as (Goodman et al. 2014). We now know that Sgr A∗ lies at FWHM ◦ h = ; (1) b = −0:046165 (Reid & Brunthaler 2004), which places 2(2 ln 2)0:5 it below the IAU-defined location of the Galactic Center where FWHM is the full width at half maximum of (although by the IAU's definition Sgr A is at the Galac- the distribution. For a given sample, we do not ex- tic center (Blaauw et al. 1960)). Goodman et al.(2014) pect significant discrepancies between the exponential investigated the implications of this offset and of the and Gaussian scale heights (Bobylev & Bajkova 2016). Sun lying above the midplane using the extremely long There are larger discrepancies between values derived \Nessie" infrared dark cloud (IRDC). Although Nessie using gas tracers compared to those derived using stellar lies below the midplane as it is currently defined, be- tracers.
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